While not limited thereto, the present invention is particularly adapted for use with so-called longwall miners. Such miners are provided with two electrical or hydraulic motors, one of which drives a winch for advancing the mining machine and the other of which drives a rotating mineral-cutting element. In a mining machine of this type, the driving energy is obtained from a three-phase supply applied to one or more electric motors through, for example, silicon-controlled rectifiers.
In mining machines of the type described above, it is known that load fluctuations will occur during the mining operation due, for instance, to differences in the hardness of the mineral being mined. Load fluctuations of this type can be constantly compared to a value corresponding to the nominal power output of the advancing motor. This is usually achieved by means of a current limiting unit which reduces the speed of the advancing motor in response to signals derived from current transformers, which measure the motor armature current, until the overload condition has disappeared. In the past, methods have also been devised for controlling mining machines in which the cutting speed of a rotating cutting element is varied as a function of the appropriate feed rate or advance rate of the mining machine. The ratio between cutting speed and feed rate in this case is fixed but variable so that irrespective of the instantaneous advance rate of the mining machine, the cutting thickness of the mineral engaged by the cutting element is the same in front of the individual cutter picks on the cutting element.
The prior art also discloses transmissions for mining machines in which the cutting tool is driven through two differential transmission branches which are parallel to each other. Advantageously, the one transmission branch, adapted to transmit a lower power level, is steplessly variable and superimposes a positive or negative rotational speed on the rotational motion of the other or main transmission branch.
It is desirable to obtain the mineral being mined with the heaviest possible cut by the cutting element because the specific cutting force diminshes with an increasing thickness of the cut. Mining machines, such as longwall mining machines, do not merely have lower energy requirements for removing a given volume of mineral, but also remove the mineral in larger pieces and thus improve its sales value while at the same time reducing the amount of dust which is caused in the mining operation.
The specific cutting work of a mining machine (i.e., the work required for obtaining a given unit volume of mineral) is defined by the ratio of the cutting element power N.sub.Wa to the mining machine feed rate V.sub.Wi. It diminishes with an increasing thickness of cut and tends toward a minimum value. This may be stated by the expression: ##EQU2## where:
A.sub.sp is the specific cutting work,
c is a constant,
N.sub.Wa is the driving power of the cutting element, and
V.sub.Wi is the winch speed.
The specific cutting work must diminish with a diminishing quotient ##EQU3## It reaches its minimum value if the differential quotient ##EQU4## becomes zero.
The input power which flows to the cutting element of the mining machine is utilized not only for cutting the mineral seam but also for discharging the mined material from the side of the cutting element, usually a drum. While the quantity of mined material obtained during the mining operation is directly proportional to the feed rate of the rotating cutting element, the proportion of power supplied to the cutting element for clearing the mined material to the side increases superproportionally because the opening, defined by the cross section of the discharge ducts of the rotating cutting element depends on the dimensions of the rotating mining element or drum and, therefore, is constant. As the feed rate increases, an increasing proportion of the driving power supplied to the cutting element will be utilized for discharging the mined material from the side of the drum, and the power required for driving the cutting element will necessarily increase. Stated in other words, if the driving power for the cutting element is constant, the portion of the total power applied which is used for discharging the mined material will diminish as feed rate increases. An increasing feed rate of the mining machine also results in an increase in the penetration depth of the cutting picks on the rotating mining element into the face section and this leads to a further increase of cutting element loading and, consequently, cutting element driving power. The increase of power resulting therefrom may also be superproportional if the feed rate of the machine increases to such an extent that not only the cutting edges of the cutter picks on the cutting element but also the cutter pick shanks penetrate into the mineral, or the cutter pick holders come into contact with the work section of the face of the mineral being mined. Under these circumstances, the driving power of the cutting element will increase superproportionally and lead to a substantial deterioration of the ratio ##EQU5## which is proportional to the specific cutting work.
Furthermore, only a small portion of the area of the mineral in front of the cutting element will be engaged by and detached by the cutting picks mounted on the circumference of the cutting element. The other portion of the mineral which remains between the rows of cutting picks on the cutting element will break off as soon as the notches cut by the cutting picks have reached a specific depth. Investigations have shown that the specific power consumption for cutting picks with uniform cutting track spacing diminishes with an increasing cutting depth under the effect of adjacent cutting tracks and reaches its optimum when a specific ratio between the thickness of the cut by the picks to the cutting pick spacing is obtained.